Function of an enzyme is to lower reaction's activation energy and to increase speed of a reaction, but to make it happen, enzyme and substrate must fit spatially. The shape of an enzyme is essential to make enzyme-substrate complex - if active site of an enzyme doesn't fit to substrate, there won't be a enzyme-catalised reaction.
RUBISCO!
Ribulose-1,5-bisphosphate carboxylase/oxygenase
I would say zygote ® fetus ® embryo ® blastocyst
Answer:
The meaning of force? or gravity pulling the object towards the Earth.